The Electromagnetic Wave Absorption Of The Complex Microwave Equivalent Parameters

With the development of radar technology, the traditional absorbing materials have been unable to meet the need at present, Iron-based nanocrystalline soft magnetic materials have become promising high-frequency magnetic materials owning to high saturation magnetization, low coercivity and obvious nano-effects. According to magnetic bounds relations, high microwave permeability and extended absorption band can be obtained for flake magnetic particles. Based on the above considerations, wet milling process is used for preparation of FeSiAl nanocrystalline flake particles. We delve into the effect of preparation technology (Speed of ball mill, Ration between balls and samples, Milling time) on the morphology, static and dynamic magnetic properties of nanocrystalline alloys and thus obtain the optimum conditions.Radar absorbing materials (RAM) are usually composites in which the absorber and matrix (adhesive) are mixed at appropriate proportion in practical application. The performance of RAM is closely related to effective permeability and permittivity of composites. Therefore, the calculations of effective electromagnetic parameters of magnetic absorbing composites have been one of the most significant basic researches, whose core contents include the investigation on resonance mechanism and expression on mixing formula. We can quickly and accurately design absorbing materials with expected electromagnetic properties on the basis of figuring out these issues. Previous studies found that aligned flake ferromagnetic particles in the matrix is advantageous to the improvement of absorbing properties. For in-depth study, commercial software CST ? microwave studio is used to investigate spatial orientation and position distribution effect on the effective permeability of composites consisting of aligned ferromagnetic flakes and Maxwell-Garnett mixing formula is improved considering these two factors. The effective permeabilities of simulation models calculated by virtue of corrected Maxwell-Garnett mixing formula are nearly consistent with simulation results. Furthermore, semi-empirical theoretical formula can successfully predict the effective permeability of composites in which the flakes are not aligned.Main resonance mechanisms of ferromagnetic nanocrystalline particles in GHz band are natural resonance and exchange resonance. Therefore, Landau-Lifshitz-Gilbert equation can be used to obtain the expression of intrinsic permeability. In this paper, we put emphasis on the investigation about the resonance expressions of ferromagnetic nanocrystalline particles with different shapes (sphere, flake, fiber), whose orientations are aligned and random in the matrix. Maxwell-Garnett mixing rule is further improved considering percolation threshold and dipole-dipole interaction between particles effect on the effective electromagnetic parameters. The effective permeabilities of flake composites for two kinds of orientation cases have been calculated and an excellent agreement between theoretical and experiment results is obtained. This means that it is possible for us to calculate effective electromagnetic parameters for different orientation cases from static magnetic parameters, which laid a theoretical foundation for the performance design of magnetic absorbing materials.